Noun
A person who hits on, flirts with, or generally acts interested in someone who is already actively being sought after in a romantic or sexual sense, drawing attention away from the original interested party to himself or herself.

I was talking with this hottie at the party when a swooper interrupted our conversation and would not leave him alone for the rest of the evening.

The person who makes the first move in new romantic relationships. Typically, one person will be the swooper or more of a swooper than the other person. The swooper is the person who hits on on the other person, asks them out on a date, reaches over to hold their hand the first time, leans in for the first kiss, etc.

Jen: Tell me about how you guys met. Who swooped who?
Maria: We knew each other for a while, but he wouldn't do anything, so I finally had to ask him out.
Rod: Yeah, she's always been the swooper.